1. Field of the Invention
The present invention relates to a steering apparatus for vehicle adapted to vary a ratio of a steered angle of steerable wheels to a steering angle of a steering member.
2. Description of Related Arts
The above steering apparatus for vehicle is shown in the following documents 1 to 3.
Document 1: Japanese Unexamined Patent Publication No. 2002-240729
Document 2: Japanese Unexamined Patent Publication No. 2004-224085
Document 3: Japanese Unexamined Patent Publication No. 2005-343205
The above document 3 discloses an arrangement comprising: a first shaft connected with a steering wheel; a second shaft connected to vehicle wheels; and a planet gear mechanism for rotation-transmittably interconnecting the first and second shafts. The planet gear mechanism comprises: a first sun gear fixed to the first shaft; a second sun gear fixed to the second shaft; a plurality of pinions disposed around the first and second shafts; and a carrier enclosing the pinions. The rotation of the steering wheel is transmitted to the first shaft, the first sun gear, the pinions, the second sun gear and the second shaft in this order and is finally transmitted to the vehicle wheels.
A transmission ratio of the planet gear mechanism (a ratio of the number of revolution of the second sun gear to the number of revolutions of the first sun gear) may be varied by a first motor driving the carrier into rotation. An output from the first motor is transmitted to the carrier via a small gear connected with an output shaft of the first motor, and a toothed portion formed on an outer periphery of the carrier and meshed with the small gear. Thus the number of revolutions of the carrier is varied.
In a case where the transmission ratio is varied in the aforementioned manner, speed shifting is carried out between the first and second shafts. As a result, a relation between the steering angle and the reaction force from the vehicle wheels is varied from that in a state prior to the change of transmission ratio. The characteristic of the reaction force varies irrespective of the intent of a driver, and a driver has an uncomfortable steering feeling. In order to eliminate this uncomfortable steering feeling, an arrangement is made such that a second motor is provided and an output from the second motor is transmitted to the first shaft via a reduction mechanism including a small gear and a large gear. Thus the variations of the characteristic of the reaction force are prevented.
In the case of the document 3, however, the first and second motors oppose to each other with the steering shaft interposed therebetween. The housings of these motors are protruded radially of the steering shaft so that the system is increased in size.
In view of the foregoing, the invention has an object to provide a steering apparatus for vehicle achieving size reduction.